In order to make an informed decision, we should probably start with a look at the rules for Geomancy. Two models in Circle have this ability: Megalith and Woldwardens. It allows you to force the model to cast one of its warlock's spells with a cost of 3 or less, using your warlock's fury stat for attack rolls, and you can't cast a spell with a RNG of SELF or CTRL. What's also important here is that the model counts as casting the spell - so that means anti-casting abilities (like counter-magic, null magic, etc.) can prevent a model from using Geomancy. The spell can also be boosted as normal, you just force the warbeast instead of spending fury on your warlock.
Let's compare this ability to Gallows Groves. While Gallows Groves also prevent models from healing or being healed and losing tough in a small radius, the ability I want to focus on with them is that a Circle warlock can channel a spell through them. This means that while the Gallows Groves are within the warlock's control area, the warlock can cast spells as if the Gallows Groves are the origin for the spell. Fairly simple compared to Geomancy, but how do these both compare?
One important thing to realize about Geomancy is that you get a huge price discount for spells. You can cast a spell that would normally cost 3 fury (half or almost half of our warlock's typical compliment of fury points) for only one, and the warbeast can then force to boost the spell on top of that. Since we have to force our warbeasts anyway to generate fury, this is a boon to us - especially since the models that have Geomancy are constructs, you can also run them "hotter" than normal without worrying about frenzy the next turn.
Another significant (and often overlooked) advantage here is that if your construct was simply engaged to tie them up, you can potentially Geomancy an offensive spell, kill the model in question, and then charge a new target! Since spells can be cast at any point in your activation (and Geomancy can also be used during your activation), it affords a greater flexibility of options. Additionally, most people don't realize that a model can cast a spell at a different target, even when engaged, meaning that you can conversely charge a target to get some extra range, and then Geomancy a spell at a completely different target!
Lastly, consider the tactical use of cycling upkeeps between targets. A model with Geomancy effectively allows three models/units to benefit from an upkeep on any given turn (upkept from previous turn on unit A, Geomancy onto unit B, warlock casts on unit C), which allows for great efficiency of certain upkeeps.
So what's the downside? They're expensive. At 9 points for a Woldwarden, these guys are a bit expensive considering they only hit with a POW 15. While they do have Chain Attack: Smite, that's a fairly fury-heavy ability to make sure it lands most of the time, and then you won't have the fury you need to do other things because these guys only have 3 fury to play around with. While Megalith is better with +1 MAT and POW, weight of stone, and 4 fury, he's still expensive and more underwhelming than other characters like Molik Kharn or Typhon. Then, add to the mix that these guys are constructs and can't be healed, and you can see why some warlocks aren't excited about Geomancy.
So given that Geomancy is so awesome, why even bother taking Gallows Groves? The first advantage is price. Each Grove is only 1 point - much more economical than the 9 or 11 points for a Woldwarden or Megalith. This means that you can have three new threat angles for your warlock for fewer points than a light warbeast. This allows you to take other warbeasts for particular tactics (be it a Feral for hitting harder or a Gnarlhorn because he has more fury to generate) while still being threatening with your spells. This is huge for certain warlocks who need to generate more fury and the Woldwarden will only generate three - and can't be healed by most warlocks, limiting his effectiveness over time. Also, unlike arc nodes, these guys don't engage enemy models (much like shifting stones) and you can always channel a spell through them, so your opponents can't just run to engage them like you can do with Cryx light warjacks.
So what's the downside? They're slow. Sure, they have a placement of 5" (like the Sentry Stone), but even with their advance deploy, they're not going to be able to cover much ground over the course of the game like other channelers (Cryx light warjacks, Legion Spell Martyrs). They also are fairly easy to kill, with DEF 5 and ARM 16, they can be brought down fairly easily even at range - since their only defensive protection is prowl. They lack speed and might have to sacrifice better positioning for survivability, so they won't be able to get the epic threat angles that Cryx or Legion can achieve - but at least ours don't pop like spell martyrs! While Entropic Force sounds great, it's low range is fairly limiting - especially when coupled with the fact that they can really only move 6" a turn and a warlock/warcaster will see them coming well before they get there.
So which will each Circle warlock prefer? Let's look at each warlock and justify whether they would prefer Geomancy, Gallows Groves, or Both.
pBaldur loves his constructs and since he often won't be casting any offensive spells, having Woldwardens to cycle Stone Skin is far more useful, especially since he's the only warlock who can also heal them.
eBaldur - Geomancy
eBaldur loves his constructs just as much! Epic Baldur can still heal constructs but his theme force does include Gallows Groves and gives them a much deeper deployment (20"), making them more threatening. Crevasse is good and all, but with only 6 fury, he'll still prefer Geomancy to spam Roots of the Earth, especially since his constructs are way better than with any other warlock.
Cassius - Both
Cassius is an interesting warlock for Circle as it is, but depending on a desired playstyle, he could equally benefit from both. Being able to cycle Curse of Shadows can be quite a boon for Cassius and effectively allow much of his army to be incorporeal for a turn. Stranglehold is also a great spell for Geomancy since a Woldwarden can boost the damage without worrying about spending too much fury on a typically mediocre spell. Gallows Groves, on the other hand, allow Cassius to actually assassinate an opponent with spells and take advantage of Black Roots without getting into harms way. Lastly, Cassius can potentially have up to 11 fury, meaning that Hellmouth can be the devastating spell it's meant to be, and he can channel it through a Gallows Grove since it's too expensive to be used by Geomancy anyway.
Grayle - Geomancy
Grayle is all about himself and solos, thanks to Storm Rager. Having a Woldwarden or Megalith around to cycle such an awesome spell is a huge boon. They can also simply save him some fury so that he can become the whirlwind of death that he should be. Also, being able to Geomancy Gallows is probably better than channeling it (ironic?).
pKaya - Gallows Groves
pKaya loves her living beasts. One could make the argument that Geomancy would be fine because she has some great fury mitigation between Soothing Song and her feat, but that's 9 points for a warbeast that's not taking advantage of the flat +2 MAT buff that she grants (which is huge). No, she could do with a Gallows Grove or two for points fillers, if for no other reason than the fact that she could potentially do a spell assassination with Spirit Fang without having to cast Spirit Door first. Even then, she'll be fine without either.
eKaya - Gallows Groves
eKaya loves her living beasts and I think with her it's an even easier choice, although she could really do without since she already has Laris. In case you're protective of Laris, I guess you could take a Gallows Grove to fill a point for an extra threat angle with Dog Pile, but really she's fine without either.
Kromac - Geomancy
Kromac is actually a tough call. While he really likes his living warbeasts, I think that taking a model with Geomancy with him often has more advantages than taking a Gallows Grove (although he can do fine without either). Having a defensive animus like the Woldwarden's or Megaliths is beneficial and being able to Geomancy his spells like Rift when you have nothing better to do could be somewhat funny. Also, Kromac likes to be a little more forward anyway, and a Gallows Grove will just encourage him to sit back longer when he should be using his threat range as a deterrent for the enemy. Again, he will do fine without either.
pKrueger - Geomancy
This can be a somewhat difficult choice, but I ultimately must side with Geomancy here, based upon my experience with pKrueger. Hell, I take two Woldwardens to throw out two Chain Lightnings per turn, which usually cleaves through enemy infantry with ease. This leaves Krueger enough fury to consistently throw out spells like Skyborne and even a Deflection if he must, and then he can always boost a shot with his own shooty lightning. Lastly, the ability to cycle Lightning Tendrils can be a huge boon on your warbeasts to spread out that extra threat range. The added flexibility and fury savings that Geomancy affords far outweighs being able to channel Tornado, in my opinion.
eKrueger - Geomancy
Similar to his primal version, I think that epic Krueger benefits more from having multiple models with Geomancy to save his fury for other things. Add to that fact that his spell list reads like a "best of Warmachine/Hordes" and they can all be cast by Geomancy, and you have no reason to take Gallows Groves, in my opinion.
Mohsar - Both (Either)
Mohsar will make great use of both, but you should choose to go either one way or the other. Geomancy will allow him to save some fury (which he needs because he has such awesome spells) and the ability to cycle Curse of Shadows, much like with Cassius, will be really useful for getting models into better positions. There's also some synergy with Megalith's Bountiful Restoration and Maltreatment That said, however, they are expensive and typically are lower fury, and Mohsar has no way of healing them. If you want to take more heavy-hitters, then Gallows Groves would be a great way to extend Mohsar's reach with his spells and still take lots of models. Mohsar is such a toolbox warlock that you can build him either way and still have some viable options, but taking both at the same time is not recommended.
Gallows Groves were made for Morvahna. Not only can she offer them the forests that they need to benefit from prowl, but they offer her the ability to really reach out and tag key models with Influence while staying back in relative safety. Fury isn't really an issue with her either, so the discount on her spells pales to the advantage of having a cheap channeler, especially when she'll want to take more infantry and fewer beasts. Lastly, the Gallows Groves might actually be able to get somewhere downfield with Morvahna, as opponents will usually be spending most of their time trying to remove other infantry models in her army while the Groves slowly creep along. Once they get there, the ability to remove healing and tough will be huge assets for a Morvahna force that has lots of infantry, or she can even use Influence again to move key models into the Groves' deathtrap.
Ultimately, I think that Gallows Groves offer some great options to our warlocks that might have needed them (Mohsar, Morvahna) but Geomancy is far from dead. I know that I will be picking up three of the suckers and I intend to build a Mohsar list using them, and maybe they'll even convince me to start playing with Morvahna or Cassius. Stay tuned!